Method for supplying current to the anode of aluminum refining cells



July 4, 1961 E. RAVIER 2,991,235

METHOD FOR SUPPLYING CURRENT TO THE ANODE 0F ALUMINUM REFINING CELLSFiled July 2, 1957 Q\ FIIIIIIIIIIIIIIIIIIIIIII'IIIIIIIIIIIIII"I'II'IIIIIIIIIllIIIllIIlIIIIIIIIIIIIII/II,

INVENTOR Emile Ravier BY 6(4/ m ATTORNEY United States PatentO METHODFOR SUPPLYING CURRENT TO THE ANODE F ALUMINUM REFINING CELLS EmileRavier, Mercus-Garrabet, France, assignor to Pechiney, Compagnie deProduits Chimiques et Electrometallurgiques, Paris, France, acorporation of France Filed July 2, 1957, Ser. No. 669,656 Claimspriority, application France July 13, 1956 3 Claims. (Cl. 204-67) Thepresent invention, which is based upon the results of applicantsresearches, relates to improvements in the method of and device forsupplying current to the anode of aluminum refining cells using thethree superposed layer process.

In this aluminum refining process by igneous electrolysis, there is useda soluble anode consisting of a molten alloy of aluminum with a heavymetal intended to impart to the alloy a suitable density so that it doesnot mix with the electrolyte layer superposed thereon, nor with therefined aluminum layer which is itself superposed on the electrolytelayer.

To supply direct current to this molten alloy, there are generally usedelectrolytic cells with conductive hearths. Such a conductive hearth isordinarily made of a carbonaceous material, prebaked amorphous carbonblocks or rammed carbon paste, in which are embedded currentconductingmetallic bars. The anodic alloy covers this hearth and the current istransmitted thereto through the carbonaceous material by means ofmetallic bars extending outside the furnace.

With flow of current, this hearth arrangement leads to a voltage dropwhich will be called anodic voltage drop in the following description.

In an 18,000 amperes electrolytic cell, there frequently occurs ananodic voltage drop of 0.5 volt corresponding to power loss of 9kilowatts per second.

A portion of this loss assists in maintaining the temperature inside theelectrolytic cells and thus constitutes useful heat, but the remainderconstitutes a waste of power which it is desirable to avoid. However, inreducing this waste of power, it'is important not to alter the thermalbalance of the electrolytic cell. 7

In order to restore the thermal balance of the electrolytic cell whenthe anodic voltage drop is reduced, the following means are available:

(1) Increase the heat insulation of the cell, thereby reducing heatlosses;

(2) Increase the working rate (current), thereby increasing productionwhile lowering power consumption;

(3) If it be impossible to increase either the heat insulation or theworking rate, it is still possible, in any event, to increase the heightof the electrolyte so as to generate more heat in the bath by the Jouleefiect.

The production will then remain unchanged but the greater interpolardistance will enable refined aluminum to be obtained more easily.Therefore, a gain in the anodic voltage drop is an advantage in allcases.

Further, in the conventional construction of hearths in aluminumrefining cells, it is almost impossible to obtain a tight joint betweenthe refractory internal walls and the carbon bottom, because the thermalexpansion coefiicient of carbon is appreciably difiterent from that ofthe refractory materials used for the walls. The result is that theanodic alloy infiltrates into the hearth and, as a consequence thereof,the mechanical strength and the heat insulation capacity of the bottomis lowered.

The present invention enables the above named difiiculties to beovercome and, in particular, makes it possible to supply current to theanodic alloy of a refining cell with a voltage drop which does notexceed 0.2 volt, without any resultant increase in the heat losses ofthe electrolytic cells as compared to those of cells provided with acarbonaceous anodic hearth.

According to the present invention, the current is brought directly tothe anodic alloy by means of conducting metal bars which are arrangedhorizontally, vertically or obliquely, and without the interposition ofcarbon between the bars and the alloy. To this end, it is necessary toselect a metallic material which does not dissolve at all, or dissolvesonly slightly, in the alloy at the normalworking temperature ofelectrolytic cells.

Iron-base alloys are suitable for this purpose. Indeed, it is known thatthe anodic alloy of modern electrolytic refining cells has an ironconcentration which is always practically equal to the saturationconcentration of that element. The iron is regularly eliminated in theform of iron-rich crystals in an anodic segregation pool. The result isthat the capacity of the anodic alloy for dissolving iron is extremelyreduced.

Moreover, there are grades of cast iron and stainless steel which areeven more suitable than the ordinary ferrou s products.

Nevertheless, applicant has obtained satisfactory results by usingcommon steel bars. After the ends of the bars have dissolved away to acertain extent, there is established an equilibrium zone between theliquid phase of the anodic alloy and the solid phase of the steel, whichzone remains stationary without interposition of solidified alloy.

The accompanying figure shows, in schematic form, a vertical sectionalview of the refining cell with an embodiment of the anodic device of thepresent invention. This embodiment is given by way of example, and notby way of limitation.

In this figure, 1 designates the metallic shell surrounding the refiningcell; 2 the refractory heat insulating lining which constitutes thewalls and bottom of the electrolytic cell; 3 is the metallic bar, forexample, a steel bar, through which current enters the anodic alloylayer 4; 5 is the electrolyte layer disposed above the anodic alloy andon which lies the refined aluminum layer 6 which forms the cathode ofthe cell. 9, 9 designate the graphite electrodes by which the currentleaves the cell; they are connected to the bar 10. 11 is the anodicsegregation well, while 12 designates the iron-rich crystals which aredeposited therein and which are removed through the upper end 13 of thewell 11 The current density in the bars must be judiciously chosen inaccordance with the desired result:

A low current density leads to a bar of large cross section and, as aconsequence thereof, to high thermal losses across the connection;

A high current density leads to a higher voltage drop and toviolentagitation-caused by electromagnetic forces-of the molten alloy againstthe ends of the bars facilitating solution of the latter.

Applicant has obtained satisfactory results with a current density inthe iron ranging between 30 and 40 amperes per square centimeter;however, these values are not given by way of limitation.

The position of the solidification front 7 which separates the moltenalloy from the connnection may, eventually, be adjusted by disposingacross the connection and the portion thereof which is outside of theelectrolytic cell-at a place where no danger existsa cooling devicehaving an adjustable rate of flow.

This arrangement is easily adapted for the temperature regulation of theelectrolytic cells with rapid response, as it enables almostinstantaneous action in lowering the temperature of the anodic alloywithout interference by the generally large thermal inertia of theelectrolytic cell, which slows down the desired temperature reductions.This arrangement can be of interest, particularly in the case where theanodic alloy contains a substantial percentage of zinc because, as isknown, the latter presents the risk of contaminating the refinedaluminum by reason of its vapor pressure, which increases rapidly whenthe temperature rises above 750 C.

In aluminum electrolytic refining cells provided with the abovedescribed anodic connection, it is possible, if desired, to keep ahearth of carbonaceous material of conventional design if it be deemednecessary to have such a hearth for starting. In this case, one canadvantageously have the carbonaceous hearth operate in parallel with thespecial connection, and this improves further the anodic voltage drop.

But it is not absolutely necessary to retain a carbonaceous hearthbecause, if the anodic alloy be initially poured into the electrolyticcell during starting, the special anodic connection described in theforegoing can function immediately and ensures, from the very beginning,flow of the current. All carbonaceous parts can then be eliminated fromthe electrolytic cell and the internal lining of the crucible can beformed in a homogeneous manner of suitable refractory materials, such asmagnesia bricks. The sturdiness of the whole structure is improved andthere is less fear of lack of tightness of the crucible.

Example In an 18,000 ampere aluminum refining cell, there was placed ananodic current supply device consisting of steel bars in which thecurrent density attained 30 amperes per square centimeter. The bars werecooled by dropping water slowly, drop by drop, upon their ends 8. Therewas obtained in this way a voltage drop of 0.19 volt between the anodicalloy and the ends of the steel bars, outside the refining cell, whilethe solidification front 7 was perfectly stabilized.

In the foregoing example, there was used an anodic alloy comprising 63%of aluminum, 28% of copper, 4% of zinc, 2% of iron, 2% of silicon, 1% ofother minor impurities. As electrolyte, there was used a bath containing60% BaCl and 40% chiolite (2AlF .3NaF), a small amount of sodiumchloride being added to this mixture.

As previously indicated, it is possible to use alloys of aluminum and ofother heavy metals, especially richer in zinc. Further, there can beused a sodium-free electrolyte bath which contains barium chloride,calcium (a) a molten layer of an alloy of the impure metal to be refinedand a heavy metal, said layer resting on the bottom of the cell andserving as the anode,

(b) a layer of a molten electrolyte, and

(c) a layer of the refined metal serving as a cathode,

the improvement of advantageously reducing in the operation of saidmethod the anodic voltage drop in the cell, which consists in: supplyingthe current to the molten anode in the cell by a metallic barconstituted of a ferrous material in direct contact with said moltenanode and regulating the strength of the current in the bar to at current density of 30 to 40 amperes per square centimeter.

2. A method according to claim 1, where the temperature of the metallicbar is regulated.

3. A method according to claim 1, wherein the metal refined is aluminum,the alloy comprises aluminum and iron, whereby the solution of the barin the molten alloy is substantially inhibited.

References Cited in the file of this patent UNITED STATES PATENTS510,276 Lyte Dec. 5, 1893 795,886 Betts Aug. 1, 1905 1,833,425 JessupNov. 24, 1931 2,213,073 McNitt Aug. 27, 1940 2,512,206 Holden et a1.June 20, 1950 2,685,566 Schmitt Aug. 3, 1954 2,866,743 Schrnitt Dec. 30,1958 FOREIGN PATENTS 593,980 Germany Mar. 7, 1934 38,159 Norway Oct. 29,1923 162,900 Australia July 3, 1952 OTHER REFERENCES (A.P.C.), SerialNo. 369,610, published May 18, 1943.

1. IN THE ELECTROLYTIC REFINING OF METALS BY THE THREELAYER METHOD IN ACELL CONTAINING IN SUPERPOSED RELATION AND IN THE ORDER NAMED, (A) AMOLTEN LAYER OF AN ALLOY OF THE IMPURE METAL TO BE REFINED AND A HEAVYMETAL, SAID LAYER RESTING ON THE BOTTOM OF THE CELL AND SERVING AS THEANODE, (B) A LAYER OF A MOLTEN ELECTROLYTE, AND (C) A LAYER OF THEREFINED METAL SERVING AS A CATHODE, THE IMPROVEMENT OF ADVANTAGEOUSLYREDUCING IN THE OPERATION OF SAID METHOD THE ANODIC VOLTAGE DROP IN THECELL, WHICH CONSISTS IN: SUPPLYING THE CURRENT TO THE MOLTEN ANODE INTHE CELL BY A METALLIC BAR CONSTITUTED OF A FERROUS MATERIAL IN DIRECTCONTACT WITH SAID MOLTEN ANODE AND REGULATING THE STRENGTH OF THECURRENT IN THE BAR TO A CURRENT DENSITY OF 30 TO 40 AMPERES PER SQUARECENTIMETER.